One important factor seems to be a chronic shortage of front-line prison staff across the prison estate.

According to research carried out by leading prison reform charity, The Howard League for Penal Reform, the number of officers fell in England and Wales to just under 14,700 in June 2016.

It’s a drop of around 420 from the previous year.

That followed an overall cut in the number of prison staff between 2010 and 2013 of around 30%.

(Picture: Getty)

At the same time, the prison population remains historically high at nearly 85,300.

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Closure of some older, smaller jails in recent years has left most inner city Category-B establishments overcrowded.

Two (occasionally even three) men are now sharing a tiny cell that was originally designed to house just one person.

Having too few prison officers on the landings not only means that it can be much more difficult to control hundreds of men safely, but staff shortages can lead to jails operating a ‘restricted regime’.

That means inmates are often left in their cells for much longer each day, leading to the cancellation of education classes, vocational training, library access, work assignments and exercise.

Even urgent medical treatment at local hospitals can be postponed.

In some prisons, ‘restricted regimes’ have been running for many months or even years with the result that cell blocks can quickly become pressure-cooker environments.

(Picture: Gaston Paris/Roger Viollet/Getty Images)

Over a period of time, such disruptions to the daily routine can fuel widespread frustration.

Even seemingly minor issues – such as a sandwich being served instead of a scheduled hot meal or the cancellation of visits by family – can lead to outbursts of anger and even full-scale prison riots.

Although there are regular disturbances and protests in prisons across England and Wales, few incidents escalate into major rioting as was seen recently at HMP Birmingham, where four wings were severely damaged.

However, HM Inspectorate of Prisons has published recent reports, which warns that an increasing number of other jails are teetering on the brink of serious trouble.

Another key ingredient in this toxic mix is the easy availability of all kinds of drugs, albeit sold by inmate dealers at a much higher price than out on the street.

Synthetic drugs – known as ‘new psychoactive substances’ or NPS – have flooded British prisons since 2012.

These now illegal chemical compounds are highly addictive. They can also have very unpredictable effects on users, unlike most traditional illegal drugs.

(Picture: Getty)

Prison staff report that some inmates who use NPS can become uncharacteristically violent, attacking other prisoners as well as officers.

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As more and more prisoners turn to illegal drugs, especially NPS, levels of debt are also increasing.

Few prisoners can maintain a drug habit on weekly prison wages of around £8-£14 and jail drug dealers are not the forgiving kind.

They charge daily interest on overdue debts that make payday loan firms look as generous as Father Christmas.